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J. Biol. Chem., Vol. 277, Issue 16, 14159-14171, April 19, 2002

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Functional Diversity of Xenopus Lymphoid Enhancer Factor/T-cell Factor Transcription Factors Relies on Combinations of Activating and Repressing Elements^*

Dietmar Gradl^§, Alexander König^¶, and Doris Wedlich^¶

From the ^¶ Abteilung Biochemie, Universität Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany,  Zoologisches Institut II, Universität Karlsruhe, P.O. Box 6980, Karlsruhe 76128, Germany

Lymphoid enhancer factor/T-cell factor (LEF/TCF) high mobility group box transcription factors are the nuclear transducers of the Wnt/-catenin signaling cascade. In Xenopus, three members of the LEF/TCF family, XLEF-1, XTCF-3, and XTCF-4, with distinct but partially overlapping expression patterns have been identified. The individual Xenopus LEF/TCF family members differ extremely in their properties of target gene regulation. We observed that in contrast to LEF-1, neither XTCF-3 nor XTCF-4 can induce secondary axis formation upon ventral overexpression in Xenopus embryos. To identify functional motifs within the LEF/TCF transcription factors responsible for target gene activation or repression, we created various mutants and a set of XLEF-1/XTCF-3 chimeras. In overexpression studies, we asked whether these constructs can mimic an activated Wnt/-catenin pathway and lead to the formation of a secondary body axis. In addition, we examined their capacity to rescue a loss-of-function phenotype given by dominant negative LEF-1 expression. We further analyzed their ability to directly activate target genes in reporter gene assays using the LEF/TCF target promoters, siamois and fibronectin. We found that a region homologous to exon IVa of human TCF-1 is an activating element. This is flanked by two small repressing motifs, LVPQ and SXXSS. Our findings implicate that the motifs identified here play an essential role in determining cell type-specific activity of LEF/TCF transcription factors.

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